Clearance of albumin by cortical bone and marrow

Male Sprague-Dawley rats, approximately 300 g, were anesthetized. The jugular vein and carotid artery were cannulated and the left femoral vein was exposed. The femoral vein was then ligated, and 2 minutes later albumin, labeled with 125I, was injected. The albumin was allowed to equilibrate in the vascular system for 3 minutes, and then 1 group of animals was euthanized. Further groups of animals were euthanized after 15, 30, and 60 minutes. Tibias were removed from both legs together with samples of muscle and liver. The tibias were separated into diaphyseal cortex, marrow, and proximal and distal ends. All tissue samples and terminal blood samples were counted for radioactivity, and volumes of distribution were calculated. In cortical bone the volume distribution increased from an initial value of 12.9 +/- 1.1 microL/g to 18.6 +/- 2.5 microL/g at 30 minutes in the control leg (mean +/- standard error of the mean, n = 5). In the congested leg, the volume of distribution increased from 10.6 +/- 0.6 to 19.6 +/- 2.1 microL/g during the same time interval. The difference in rate of increase between the congested and control leg was not statistically significant. Albumin seemed to equilibrate within 3 minutes within the marrow. The data are consistent with the hypothesis that macromolecular transport in the interstitial fluid of cortical bone is coupled to convectional flow from endosteal to periosteal surface. This data may help to understand mechanisms by which venous hypertension has been shown to stimulate bone growth and fracture repair, and also the distribution of bone density changes observed after prolonged exposure to microgravity.